1. Field of the Invention
The present invention relates to wireless communication devices and methods. More particularly, the present invention relates to a wireless communication device for performing wireless communication, a wireless communication device for relaying data exchanged between a base station and a terminal and transmitting control information to the terminal, and a wireless communication method for a wireless communication system including a base station, terminals, a non-controlling relay station for transmitting data only and a controlling relay station for transmitting both data and control information.
2. Description of the Related Art
In wireless communication systems, communication is generally carried out between a wireless base station, which is connected to a higher-level network by a wired link, and a terminal. There have also been known systems in which a relay station for relaying data by wireless is introduced between a wireless base station and a terminal. By introducing relay stations, it is possible to expand the coverage of communications and improve the communication throughput of terminals.
FIG. 18 illustrates a wireless communication system configured for wireless relay communications. As illustrated, the system includes a base station (BS) 101, a relay station (RS) 102, and a mobile station (MS) 103, which individually perform wireless communications.
For the base station 101, the relay station 102 operates as if it were a mobile station, and for the mobile station 103, the relay station 102 operates as if it were a base station. The relay station 102 receives a radio signal from the base station 101 or the mobile station 103, then performs the necessary process on the received signal, and transmits the processed signal to the mobile station 103 or the base station 101. The base station 101 and the mobile station 103 may be connected via a single relay station (single-hop connection) or two or more relay stations (multi-hop connection).
A relay communication scheme to be adopted in such a wireless communication system may be the one defined by IEEE 802.16 (WiMAX), for example, and the standardization is under way especially in 802.16j as Mobile Multihop Relay (MMR). “Relay station” is so named in 802.16j.
There are two conceivable types of communication system using relay stations.
FIG. 19 illustrates the first type of communication system. The figure shows a base station 111, a relay station 112, and mobile stations 113 and 114, together with a communication coverage area 121 of the base station 111 and a communication coverage area 122 of the relay station 112.
In the first type of communication system, the relay station 112 does not transmit control information (e.g., preamble, map information, etc.) for configuring wireless frames, and relays only data to or from the mobile station 114. This enables fast communications while restraining propagation loss.
Since the relay station 112 transmits no control information, the mobile station 114 is unaware of the presence of the relay station 112 and operates as if it were communicating directly with the base station 111. This communication system operates properly on condition that the mobile station 114 is located in an area where it can always receive control information from the base station 111.
FIG. 20 illustrates the second type of communication system. The figure shows a base station 131, a relay station 132, and a mobile station 133, along with a communication coverage area 141 of the base station 131 and a communication coverage area 142 of the relay station 132.
In the second type of communication system, the mobile station 133 may be located outside the coverage area 142 of the base station 131 (i.e., outside the area in which the mobile station 133 can receive control information from the base station 131). In this case, the relay station 132 relays data communicated between the mobile station 133, which is within the coverage area 142 of the relay station 132, and the base station 131.
Accordingly, the mobile station 133, which is located outside the area 141 of the base station 131 but within the area 142 of the relay station 132, communicates with the relay station 132 as if it were communicating with the base station 131. Thus, even though the mobile station 133 is located in an area where the control information from the base station 131 does not reach, it can communicate with the base station 131 by receiving the control information from the relay station 132.
In the above two types of communication system, relay stations are used for respective different purposes, and therefore, it is conceivable that the two different types of relay station will be used in combination. In this case, the base station needs to be configured so as to accommodate the two different types of relay station. However, such configuration has not been devised yet.
In the second type of communication system, the relay station needs to receive control information from the base station and then transmit the control information to the mobile station. However, in time-division multiplexing systems wherein data transmission and data reception cannot be carried out at the same time, the timing at which the relay station receives control information from the base station differs from the timing at which the relay station transmits the control information to the mobile station. The mobile station communicating with the relay station detects frame timing on the basis of the control information from the relay station, and therefore, the detected frame timing is different from that of the base station.
On the other hand, in the first type of communication system, the relay station itself does not transmit control information, and accordingly, the frame timing of the mobile station communicating with the relay station is identical with that of the mobile station communicating directly with the base station. Thus, the frame timing of the mobile station communicating with the relay station that transmits control information is different from the frame timing of the mobile station communicating with the relay station that does not transmit control information.
In the case of a communication system in which the two different types of relay station coexist but are serviced separately, a situation can arise where, while a mobile station communicating with the relay station that transmits control information is receiving data by using the resource specified by the relay station, a nearby mobile station communicating with the relay station that does not transmit control information transmits data with the same resource specified by the base station. In such cases, the signal transmitted from the mobile station communicating with the relay station that does not transmit control information interferes with the data reception of the mobile station communicating with the relay station that transmits control information, making the latter mobile station fail to communicate with satisfactory quality.
It is also probable that a certain relay station transmits control information during the data reception of another relay station that does not transmit control information. Generally, the control information is transmitted with high power, and thus, the relay station that does not transmit control information suffers substantial interference.
In this manner, where relay stations that transmit control information and relay stations that do not transmit control information are used in combination, an interference problem arises when multiple mobile stations simultaneously transmit and receive signals or when multiple relay stations simultaneously transmit and receive data.